1. Field of the Invention
The disclosure relates to a process for making titanium dioxide. The disclosure additionally relates to contacting the titanium dioxide with silicon tetrachloride to form surface-treated titanium dioxide. Yet additionally, the disclosure relates to using an analyzer to control the conditions for making titanium dioxide or contacting the titanium dioxide with silicon tetrachloride or both to improve the process by optimizing process efficiency.
2. Description of the Related Art
The chloride process for making titanium dioxide pigment is well known. In the chloride process, TiCl4 resulting from chlorination of titanium-containing material such as rutile ore is oxidized to form TiO2 particles. The chloride process is described in greater detail in “The Pigment Handbook”, Vol. 1, 2nd Ed., John Wiley & Sons, N.Y. (1988), the teachings of which are incorporated herein by reference.
A process for making a durable grade titanium dioxide pigment, without the necessity of depositing surface treatments on the titanium dioxide by wet treatment has been described in U.S. Pat. No. 5,562,764 of Gonzalez and U.S. Pat. No. 7,029,648 of Subramanian et al. In these disclosures silicon tetrachloride is contacted with a titanium dioxide stream at one or more points downstream from the point where titanium tetrachloride and oxygen are contacted to form the titanium dioxide. The resulting titanium dioxide product has a surface treatment of a silicon-containing material which enhances durability.
In the oxidation of titanium tetrachloride to form titanium dioxide, it is useful to optimize the conditions to avoid having titanium tetrachloride present in the product stream. The titanium tetrachloride in the product can lead to a variety of problems. In particular, if water is present in downstream operations it can convert titanium tetrachloride to titanium oxychloride species which can cause severe processing problems including “bag filter blinding” in which titanium oxychlorides clog filter pores in the pigment separation process, and blower fouling in which titanium oxychlorides accumulate on blower internals causing a lower compression ratio. In addition titanium tetrachloride can cause jet pluggage in which titanium tetrachloride in the feed jets cause poor gas distribution in the chlorinator. Additionally, the resulting titanium dioxide pigment product can have poor acid solubility because soluble titania present in the product can impair silica deposition during aqueous treatments to improve the acid solubility properties of the pigment. To avoid these problems, operators tend to utilize excessive heat during the oxidation step which is wasteful.
In the step of contacting the titanium dioxide product stream with silicon tetrachloride it has been found that if the temperature is too low silicon tetrachloride can remain unreacted. If the temperature is too high the quality of the resulting at least partially encapsulated product is poor either the encapsulation layer is fragile or many particles are not fully encapsulated leading to a product which lacks suitable durability.
The process of this disclosure overcomes these problems by integrating the process with an analyzer capable of analyzing for titanium tetrachloride or silicon tetrachloride or both.